Packaged semiconductor dies, including memory chips, microprocessor chips, and imager chips, typically include a semiconductor die, mounted on a substrate and (optionally) encased in a plastic protective covering. The die includes functional features, such as memory cells, processor circuits, and/or imager devices, as well as bond pads electrically connected to these functional features. The bond pads can be electrically connected to outside terminals to allow the die to communicate with other devices, e.g., higher level circuitry.
Semiconductor die packages typically include an underfill material between the die and the substrate. The underfill material prevents moisture and other foreign matter from entering the space between the die and the substrate, and can also aid in adhering the die to the substrate. A conventional process for attaching a die to a substrate is shown in FIGS. 1A and 1B. Beginning with FIG. 1A, a conventional die 11 includes downwardly-extending conductive pillars 16 to provide electrical and thermal connections with a corresponding substrate 30. The substrate 30 can include a cavity 34 positioned to facilitate densely packed electrically conductive connections to the pillars 16. Accordingly, the substrate 30 can include first bond pads 33a within the cavity 34 and second bond pads 33b outside the cavity 34. An underfill material 40 is applied to the underside of the die before the die 11 is connected to the substrate 30.
Referring next FIG. 1B, the die 11 and the substrate 30 are brought together. Individual pillars 16 are connected to corresponding first bond pads 33a or second bond pads 33b. During a reflow process, electrical and/or thermal connections are formed between the bond pads and the pillars, and the underfill material is distributed in the region between the die 11 and the substrate 30. The process typically includes applying pressure to the substrate 30 and/or the die 11 to fill the cavity 34 with the underfill material 40. Despite this approach, voids 42 can form in the cavity 34, which compromises the integrity of the sealing function performed by the underfill material 40. In addition, excess underfill material 41 typically squeezes out around the edges of the die 11, which can interfere with subsequent manufacturing processes, including placing other dies adjacent to or on top of the die shown in FIG. 1B.